Coronary heart disease accounts for more deaths annually than any other disease, including all forms of cancer combined. Epidemiologic studies have established that the higher the total plasma cholesterol and low density lipoprotein cholesterol levels, the greater the risk that coronary heart disease will develop.
The low density lipoproteins ("LDL") are those which float in the density range between 1.019 and 1.063 g/ml upon ultracentrifugation. They are composed of about 75% lipid (primarily cholesteryl esters, cholesterol and phospholipids) and 25% protein. Apolipoprotein B, which has a molecular weight of 510,000-dalton, is the principal protein component of LDL.
Under electron microscopy, LDL appear to be globular particles 20-25 nanometers in diameter. Each LDL particle contains approximately 1500 molecules of cholesteryl ester in an oily core that is shielded from aqueous plasma by a hydrophilic coat composed of phospholipids, unesterified cholesterol, and apolipoprotein B. Approximately 65-70% of the total cholesterol is transported in LDL. Thus, removal of LDL leads to a concomitant decrease in cholesterol levels.
Cholesterol and LDL levels are high in many people who have a high dietary intake of cholesterol or fats, and in people with a variety of familial hyperlipidemias including the uncommon, but not rare group who have inherited a genetic defect in the cell surface receptor for LDL.
Total plasma cholesterol and LDL cholesterol levels may be reduced by diet or drugs. However, drug therapy may have potential severe side effects which limit its use. More specifically, drugs that decrease cholesterol synthesis (e.g., lovastatin) have the potential to cause liver injury, cataracts, and fetal abnormalities. Further, many familial hypercholesterolemias (all homozygous types and some heterozygous types) are resistant to diet and drug therapy. Also, drug therapy usually lowers the level of high density lipoproteins ("HDL"), as well as that of LDL, and the former are thought to be protective against atherosclerosis.
Plasmapheresis, the direct removal of the patient's high-cholesterol plasma and replacement with a low-cholesterol fluid, causes only a temporary decrease in HDL levels and is a successful therapy. However, plasmapheresis is extremely expensive. Moreover, this therapy may cause transmission of contagious disease and other complications because of the long-term administration of a large quantity of human plasma products.
Specific removal of LDL only, which obviates the need for replacement with plasma, can be accomplished by using affinity chromatography equipment such as polyanion columns or anti-LDL antibody columns. However, this technique usually requires a two column system; one column removes the LDL from the plasma while the other column is being regenerated. The limited capacity of the adsorbents makes this technique cumbersome and expensive.
LDL in the blood can also be preferentially removed by its reaction with phospholipase C or D, which renders the LDL particles capable of being separated from other blood components by filtration or centrifugation. This method is disclosed in U.S. patent application Ser. No. 241,067 filed Sep. 6, 1988. While this therapy is rather inexpensive, its use is limited to extracorporeal applications.
It has recently been found that LDL modified by the enzyme phospholipase Az is rapidly cleared from the blood pool by the in vivo catabolic processes. As described in U.S. patent application Ser. No. 101,262 filed Sep. 25, 1987, reactors containing immobilized phospholipase A.sub.2 are used for extracorporeal or intracorporeal treatment to lower plasma LDL levels. One major drawback about this approach is that it requires that immobilized phospholipase A.sub.2 be used. It thus imposes a great restraint on the technical design of therapeutic systems based on the concept of in vivo clearance of modified LDL. More specifically, an immobilized enzyme-containing reactor has to be connected to the patient, either extracorporeally or intracorporeally, during treatment. Also, the immobilization process often significantly reduces the enzyme activities.